1. Field of the Invention
The present invention relates to a heat dissipation assembly and a method for producing the same. In particular, the present invention relates to a heat dissipation assembly and a method for producing the same, in which heat from a heat generator such as a semiconductor device or the like is effectively dissipated while an electrically insulating condition is maintained, and superior durability for usage for a long period of time is exhibited.
2. Description of Related Art
Heretofore, various heat dissipation assemblies for cooling a heat generator such as a semiconductor device or the like have been proposed. FIG. 7 and FIG. 8 show one embodiment of a conventional heat dissipation assembly in a semiconductor device. In FIG. 7 and FIG. 8, a bonding wire for power distribution and a sealing resin are not shown.
In the heat dissipation assembly shown in FIG. 7, semiconductor elements 11 are fixed to a resin molding part 13 of a semiconductor device 1, together with electrode plates 12. As shown in FIG. 8, the electrode plates 12 of the semiconductor elements 11 are exposed at the surface of the resin molding part 13. To the molding part 13, holes 14 for fixing are formed, and the molding part 13 is fixed to a heat sink 2 made of metal by means of fixing screws 3 inserted in the holes 14. Inside of the metallic heat sink 2, coolant passages 5 are formed, and a coolant for heat dissipation circulates in the coolant passages.
Between the resin molding part 13 and the metallic heat sink 2, a sheet 4 having insulating and heat dissipating effects is interposed, and thereby, the electrode plates 12 on the surface of the resin molding part 13 are electrically insulated from the metallic heat sink 2. In addition, the sheet 4 is integrated with the metallic heat sink 2 and the resin molding part 13 of the semiconductor device 1 by means of fixing screws 3. For this reason, heat generated in the semiconductor element 11 is dissipated via the electrode plate 12 to the sheet 4, then to the metallic heat sink 2, and then to the coolant passage 5.
As the sheet 4, in general, an electrically insulating/thermally conductive sheet which is formed from a base material having an electrically insulating property such as a silicone rubber or the like, and a thermally conductive filler, and which exhibits both functions of electrically insulating properties and thermally conductive properties is employed.
In the heat dissipation assembly shown in FIG. 7 and FIG. 8, in order to improve cooling efficiency by reducing thermal resistance of the sheet 4, it is believed that, for example, a charging ratio of the thermally conductive filler in the sheet 4 may be increased or that a contacting ratio between the surface of the sheet 4 and the surfaces having microirregularities of the metallic heat sink 2 and the semiconductor device 1 (electrode plates 12) may be improved by increasing flexibility of the sheet 4.
However, the two methods for reducing thermal resistance described above are in an incompatible relationship. For this reason, reducing thermal resistance due to making the sheet 4 flexible is limited. Therefore, as described in Japanese Unexamined Patent Application, First Publication No. H11-135691, it is proposed that an electrically insulating/thermally conductive grease 6 produced by mixing a base material having electrically insulating properties such as a silicone oil and the like, with a thermally conductive filler, is applied to the surface of the sheet 4. FIG. 9 is an enlarged view showing the contacting condition of the sheet 4 with the metallic heat sink 2 and with the electrode plate 12 in the case of applying the grease 6 to both surfaces of the sheet 4. As shown in FIG. 9, even if flexibility of the sheet 4 is not sufficient, and it is impossible to sufficiently follow the microirregularities on the surfaces of the metallic heat sink 2 and the electrode plates 12, the grease 6 is charged in the microgap between the contacting surfaces of the sheet 4 and of the electrode plates 12 and the metallic heat sink 2, and for this reason, thermal resistance can be reduced.
According to Japanese Unexamined Patent Application, First Publication No. H11-135691, as shown in FIG. 9, it is mentioned that the sheet 4 in which the grease 6 is applied to the surfaces of the sheet can be used for a long period of time under the thermal conditions during operation of the semiconductor element. However, in practice, when the sheet 4 in which the grease 6 is applied to the surfaces thereof is used for a long period of time under increased temperatures such as the temperatures during operating the semiconductor elements, the grease 6 is denatured and is solidified, and the sheet 4 may lose elasticity, and finally, the electrically insulating properties of the sheet 4 are lost. Therefore, there is a problem in that insulating defects of the semiconductor device 1 (electrode plates 12) occur.
The mechanism of occurrences of insulating defects is described by way of FIG. 10. In a heat dissipation assembly shown in FIG. 10 in which a sheet 4 in which a grease 6 is applied to the both surfaces thereof is provided between a semiconductor device 1 and a metallic heat sink 2, the grease 6 comprises a dimethylpolysiloxane-based silicone oil base material 61 and a thermally conductive filler 62, and the sheet 4 comprises a dimethylpolysiloxane-based silicone rubber base material 41 and a thermally conductive filler 42. The sheet 4 and the grease 6 respectively exhibit superior thermal resistance. By employing a combination of these, the heat dissipation assembly shown in FIG. 10A and FIG. 10B can have superior insulating properties and heat dissipation properties under the initial conditions shown in FIG. 10A.
However, in the heat dissipation assembly shown in the drawing, the base material of the sheet 4 and the base material of the grease 6 are dimethylpolysiloxane-based materials, and they have similar chemical structures. Therefore, as shown in FIG. 10B, during use for a long period of time, the components of the dimethylpolysiloxane-based silicone oil base material 61 in the grease 6 penetrate into the sheet 4, and thereby, the sheet 4 swells, and strength, elongation properties, and the like, are deteriorated. In addition, in the grease 6 on the surface of the sheet 4, the thermally conductive filler 62 which is a solid component remains at a high rate. As a result, almost all the grease 6 is occupied by the filler 62, and therefore, the grease 6 becomes solid.
By solidifying the grease 6, the sheet 4 is fixed to the semiconductor device 1 and the heat sink 2 made of metal. However, when under the condition described above, operations of the semiconductor element of the semiconductor device 1 are repealed, since the thermal expansion coefficient of the semiconductor device 1 is different from that of the heat sink 2 made of metal, different displacements are exerted on the both surfaces of the sheet 4 in the heat cycle of the high temperature condition during operation and the low temperature condition during stopping the operation, thereby generating a force which tears the sheet 4. The heat cycle described above may occur due to not only repeated operations of the semiconductor element, but also due to setting conditions of the semiconductor device (such as in automobiles) Tf the occurrence of the tearing force is repeated during use over a long time, cracking occurs at a part of the sheet 4. As a result, between the semiconductor device 1 and the metallic heat sink 2, insulating defects occur.
FIG. 10 shows an embodiment in which the grease 6 is applied to both surfaces of the sheet 4. The problems of insulating defects described above also occur in the same manner as described above, in the case of applying the grease 6 to one of the surfaces of the sheet 4.